Three Stage Combustor For Low Quality Fuels

ABSTRACT

A combustor for low quality fuels has a devolatilization chamber receiving the fuel and separating the fuel into char and gases. Char from the devolatilization chamber exits through a first port connected to a char chamber. The char chamber reduced the char to gases and ash. Gases generated in both the devolatilization chamber and char chamber are sent to gas and particulate combustion chamber, such as a fluidized bed. The various stages are operated at the optimum temperatures for the contituents provided to that stage. The resulting process has reduced emissions.

This application claims benefit of provisional application No.61/522,976, filed on Aug. 12, 2011, the entire contents of which areherein incorporated by reference.

BACKGROUND OF THE INVENTION

Low grade fuels, such as coal, burn in three stages including volatilerelease, char combustion, and gas and residual particulate combustion.Most conventional systems attempt to perform all three stages in asingle module, such as the cyclone combustor depicted in FIG. 1, whilesome advanced systems have a first fuel rich chamber followed by asecond fuel lean chamber. The control of the air-fuel mixture reducesNOx emissions by lowering peak temperature.

Coal-fired power plants generate nearly half of the electricity in theUnited States. The other main energy sources are the fossil fuelsnatural gas and oil; nuclear power; hydroelectric power; and therenewable sources including solar, wind, geothermal, tidal, and biomasssources.

There are major problems with each of these sources. Fossil fuelsupplies are being depleted, while concerns about carbon dioxide arerising. Nuclear technology is expensive, and there are concerns aboutthe safety of both the plants and the spent fuel rods. Hydroelectricpower is limited to areas with adequate water supplies and acceptableenvironmental impacts. Green technologies are promising, but thesesources are not yet capable of supplying a major portion of the nation'spower needs. Beyond these problems, there is increasing world-widecompetition for power, particularly in China and India. The net resultis that all power sources must be considered to meet growing demand.

As this demand grows, the relative importance of these various sourcesis changing. In particular, there is strong pressure to decrease the useof coal in the Unites States. Even under the most severe forecasts,however, coal will remain a major contributor to the nation's energysupply at least for the next several decades.

The major objection to coal use is environmental impact: the burning ofcoal releases large amounts of emissions. To maintain or even increasethe use of coal, highly effective emissions controls are thereforenecessary. Unfortunately, coal emissions are extremely complex, and aretherefore expensive and difficult to control.

The underlying problem is the structure of the coal itself. Coalconsists mostly of carbon, which is why coal is black. In addition tocarbon, coal also contains a mixture of highly reactive carbon compoundscalled volatiles. Finally, coal also contains inert mineral matter thatwas laid down when the seam was formed. Coal therefore contains some ofevery chemical element on earth.

This complex structure in turn yields a complicated combustion process.The first step is the emission and burning of the volatiles. The loss ofthe volatiles leaves char, which burns more slowly but quite intensely.The loss of the char then leaves the mineral matter, in the form of ash.

In practical systems, however, the combustion is not complete, leavingunburned fuel, volatile organic compounds, and soot. There are alsoinorganic gas emissions, notably oxides of nitrogen and sulfur. Finally,the mineral matter can form solids that are suspended in the exhaustgas, as well as vaporized, highly toxic metals, notably mercury.

An effective emissions control strategy must therefore address all ofthese complex, diverse problems. Without such a process, it will not bepossible to use the nation's plentiful coal reserves.

One approach is to remove the coal contaminants before combustion.Without contaminants such as mineral matter, the “clean coal” productcan thus burn with minimum emissions. Unfortunately, advanced coalcleaning processes are expensive and difficult, thereby requiringsubsidies or other incentives. Pre-combustion clean coal technologiesare therefore not currently in general use.

Because it is thus not practical to clean the coal before combustion,the only alternative is to burn the coal, and clean the emissions duringand/or after the combustion process. This approach includes advancedcombustor designs; electrostatic precipitators; scrubbers for sulfuroxides; fabric filters; catalysts; and various additives for selectedproducts, notably mercury and nitrogen oxides.

The overall result is that “clean coal” today essentially amounts to amore or less a conventional combustor 2, seen in FIG. 1, having a slagtrap 3 at the bottom, and provided with an exhaust 4 and an inlet 5through which fuel enters, followed by a complex array of technologieseach directed at a separate emission. A flowchart of a typical system isdepicted in FIG. 2, where the combustor is followed by a mercury trap,an electrostatic precipitator, nitrous oxide and sulfur oxide treatment.

As regulations tighten, the existing technologies must become more andmore effective, and thus more and more expensive. Furthermore, as EPAimposes new regulations, more components become necessary: mercurycontrols are currently under way, and selenium control is not farbehind. This increasingly complex and expensive piecewise approach toemissions control is simply not sustainable. Another approach istherefore necessary.

It is an object of the invention to provide a combustor for low qualityfuels having reduced emissions.

It is another object of the invention to provide a three stage combustorfor separating the fuel into components.

It is still another object of the invention to provide a first stagecombustor separating coal into char and small particles.

It is yet another object of the invention to provide a three stagecombustor having a devolatilization chamber, a cyclone combustor and afluidized bed.

These and other objects of the invention will become apparent to thoseof ordinary skill in the art after reading the disclosure of theinvention.

SUMMARY OF THE INVENTION

The system controls oxygen and fuel mixtures by having three distinctcomponents, one for each of the three stages. Under this approach, thecombustion process can be matched to the specific characteristics of thefuel component being used.

The first two components preferably use “entrained flow” conditions, aparticularly effective technique in which the coal is ground so finelythat it flows in the combustion air stream. An enhancement of this basicentrained flow system is a sudden expansion, which slows and mixes theair and fuel, thus improving combustion. A conical expansion zone iscalled a “quarl.” If the quarl stops before reaching the outside wall, a“recirculation” zone forms between the quarl end and the outer wall. Thespinning hot gases in this zone help to stabilize the flame.

A further enhancement is to rotate the incoming air to produce a“swirl.” At sufficiently high swirl numbers, the combusting fuel and airmixture produces a strong central recirculation zone that extends intothe combustor body. This recirculation zone provides rapid fuel heating,thereby greatly aiding combustion stability and efficiency.

The third component preferably uses “fluidized bed” conditions, aparticularly effective technique in which the fuel is burned in anupflowing stream of gas, typically including a non-combusting fillmaterial. In this application, the fill material is preferably limestonefor sulfur capture.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of a conventional cyclone combustor;

FIG. 2 is a flowchart of a conventional power process;

FIG. 3 is a cross sectional view of the devolatilization chamber;

FIG. 4 is a cross sectional view of a powder ash cyclone combustor usedin the invention;

FIG. 5 is cross sectional view of a slagging embodiment of the cyclonecombustor used in the invention;

FIG. 6 is a side schematic view of the first two components of thecombustor;

FIG. 7 is a schematic view of the third component;

FIG. 8 is a top schematic view of the first two components of thecombustor connected to the third component;

FIG. 9 is a flowchart of conventional power system, with additionalequipment necessary to meet new regulations; and

FIG. 10 is a flowchart of a power system having a three stage combustor.

DETAILED DESCRIPTION OF THE INVENTION

1. Devolatilization Module

The first component is a devolatilization module 10 coupled to a secondchar extraction/volatile collection module. The devolatilization moduleproduces two separate streams. A first stream is a partially burned,highly fuel rich volatile stream for later combustion, as will beexplained later. This stream contains most of the gas yield. A secondstream contains concentrated, partially burned char particles. Thisstream contains minimum gas phase products that are burned in the charcombustor module, also to be explained later.

As seen in FIG. 3, the devolatilization module has an inlet 11 allowingthe introduction of air and fuel followed by a quarl zone 12 andrecirculation zones 13. The devolatilization module has two sections toform two separate streams; a volatile generation/combustion section 14,coupled to a subsequent char extraction/volatile collection section 15.The inner surface of the devolatilization module can be refractory linedto enable the internal temperatures to reach high levels, adding to thestability of the generated flame. For compact, low cost systems, the twosections can be joined across a single, straight wall. However, theoutput streams of the two section system are better separated than theoutput streams of a single section system.

a. Volatile Generation/Combustion Section

The Volatile Generation/Combustion Section 14 produces maximum volatileflame stability to prevent a flame-out and maximum volatile yield (whichalso yields minimum NOx) by constricting the combustor at the end of thevolatile flame front (essentially a mirror image of the quarl zone),thus yielding a compression of the central recirculation zone and amounting point for matched antennae for electromagnetic (microwave)enhancement of the flame. Microwaves have been used in other types ofsystems to extend the lean limits of combustion, in an attempt to limitNOx. The system uses microwaves to extend the limits to the oppositeend, to make the most fuel rich system possible for maximum volatileyield. A sonic amplifier section couples directly to sonic drivers. Likemicrowaves, sound is also known to influence flames. In particular, lowhundreds of Hz sound is quite effective at stabilizing poorly mixedflames. Orthogonal sources of low kHz sound and upper tens of kHz soundare also quite useful in the presence of particles. One set of sonicdrivers is diametrically opposed to another and emits a first frequencywhile a second set of sonic drivers is diametrically opposed to anotherand emits a second frequency. The four sonic drivers can be evenlydistributed about the char chamber 14. Combinations of microwaves andsound, or no microwaves and no sound can be used.

This section produces maximum particle separation to the periphery,thereby yielding maximum particle loading to the subsequent char chamber20 through a first outlet, or side port, 16, and minimum particles andgases to the subsequent gas phase combustor through a second outlet, orend port, 18 and ideal mixing of combustion and separation air for thesecond section of the devolatilization chamber.

b. Char Extraction/Volatile Collection Section.

Air is added to the output of the Volatile Generation/Combustion Section14 to keep the char burning, but only slightly, thereby keeping any ashparticles in a dry, powder (not melted) state. The char extractionchamber expands rapidly, with the first outlet 16 in a side wallremoving the particles with minimum amounts of carrier gas. This feedsthe char combustor 20, described later. The far end of the chamber 15 isconfined to produce a cyclone action resulting in maximum particlecollection and maximum cleaning of the volatile stream. The volatilestream is extracted along an second outlet 18 along the central axis.Sound may be applied along the central axis to enhance flame stability,keep particles out of the exhaust stream, and disrupt soot formation.

2. Char Combustion Module

The char that emerges from the devolatilization phase is essentiallysolid carbon. Solid carbon burns by first forming carbon monoxide at thesurface, which then burns quickly. Char combustion is thus inherently aslower process than volatile combustion.

The immediate problem is to match the speeds of the devolatilization andchar combustion steps. Conventional systems try to perform bothfunctions in a single unit, with the result that neither function isperformed well. Instead, the system uses separate chambers to performthese functions.

A particularly effective means of burning char is a cyclone combustor20, as depicted in FIG. 4. In this special case of entrained flow, theparticles are thrown to the wall after being introduced through an inlet23. The burning of the char creates gases which exit the cyclonecombustor through an outlet 22 and ash, which falls to the bottom of thecyclone combustor. At sufficiently high temperatures, the ash can meltto form liquid “slag.” Although highly effective, cyclones are notcurrently used for several reasons.

First, cyclones often have unsteady flames, leading to difficultcontrol, poor efficiency, and excessive emissions. The underlyingproblem is that conventional systems introduce the raw fuel into ahighly turbulent mixture of extremely hot gases. The fuel thereforeburns rapidly but erratically.

The char combustor 20 uses several unique flame stabilization techniquesto improve the combustion process. First, the system feeds the cyclonecombustor with char, not raw coal. Under this approach, there are novolatiles to compete with the char for the available oxygen. Therefore,the characteristic flickering and popping of conventional systemstransitions to a steady, uniform glow, indicating the desired stablecombustion. The cyclone chamber may have a bulbous bottom 24, seen inFIG. 5 allowing for the introduction of secondary combustion air throughinlet 25 and a microwave antenna 26 and/or a sonic driver to acceleratecombustion and thus melt the ash into slag, also as seen in FIG. 5.

For maximum benefit, the char fuel is introduced in a unique geometry,seen in FIG. 6. The key feature here is the ability to match flowdirections. Specifically, the output of the devolatilization chamber 10is rotating rapidly. To match this incoming flow, the flow in the charcombustor chamber rotates in the opposite direction. For instance, thedevolatilization module has a counter-clockwise flow and the charcombustion chamber has a clockwise flow in FIG. 6. The two flows thusmerge smoothly along an “S” path: counterclockwise at the top mergeswith clockwise at the bottom.

The overall geometry is two or more parallel tubes, as shown in FIG. 6.In most cases, the devolatilization chamber 10 will be vertical-up, andthe char chamber 20 will be vertical-down. Other geometries (horizontal,devolatilization vertical-down, etc.) are also possible for specificfuels. For example, coal can be used in all configurations, but a Kraftdevolatilization chamber (for paper pulp waste combustion) must beconfigured vertically upward; otherwise, a flame-out would result duringthe entry of water into the char combustor, which would cause anexplosion.

In addition to orientation, the relative sizes of the devolatilizationand char combustion chambers are also important. Again, the underlyingproblem is that devolatilization and volatile combustion is fast, butchar combustion is slow. Devolatilization chambers 10 can thus be small,but char combustors 20 must be relatively large. In conventional utilitysystems, this difference causes scaling problems in the design phase.Furthermore, this difference also causes turn-down problems whilefollowing varying loads in the operational phase.

To avoid these problems, the system may use multiple devolatilizationchambers 10 to feed a single char combustion chamber 20. Thisarrangement is easily scaled to any desired size, with no loss ofeffectiveness or efficiency. Furthermore, during operation, some of thedevolatilization chambers can be shut down at part load, thus therebyproviding unmatched turn-down capability.

The second major problem with conventional cyclones is that they produceexcessive levels of nitrogen oxides, or NOx. This problem is so severethat cyclone combustors are not in common use in the United States. Thechar combustion chamber has several means of reducing NOx in thecyclone. These techniques follow from the well-known NOx formationmechanisms. First, because the volatiles contain most of the fuel boundnitrogen, and these exit through the end port 18, not the side port 16,the char combustor produces essentially no NOx from fuel bound nitrogen.The fuel bound nitrogen NOx from the volatile stage is addressed in thefollowing fluidized bed module. Likewise, this absence of volatilesvirtually eliminates prompt NOx.

The majority of NOx is therefore thermally generated. To reduce thiscomponent, the system uses two features. First, the reactivity of charis much less than the reactivity of volatiles. Therefore, the combustionof char is relatively mild, thus generating relatively little thermalNOx. Second, the char chamber is operated at fuel rich, or low oxygen,conditions. This arrangement yields low temperatures that reduce thermalNOx. Furthermore, reducing conditions convert much of any generated NOxback to nitrogen and oxygen. This “reburn” technique is continued in thethird stage, as discussed later.

Finally, the system also uses staging to introduce the oxygen inmultiple steps, thereby avoiding local zones of excessive heat. Whilesome conventional systems use staging, only the unit has an optionalsegregated stage for complete combustion, followed by a return paththrough a reducing atmosphere.

In addition to NOx control, the char combustion chamber provides theoption of high temperature combustion, specifically temperaturessufficient to melt the waste ash into slag. Currently, coal ash istypically produced in a dry powder form. Although some ash is used forcement manufacture or other purposes, most ash is simply dumped inlandfills or collected in sludge ponds. In these locations, the ash caneither escape in bulk, or leach out into the water supply. In eithercase, the net result is severe environmental pollution, primarily due tocontamination by mercury, selenium, and other heavy metals in the ash.Melting the ash, which also contains silica, instead produces avitrified form that traps these toxins.

While other systems can produce slag, only this system has theprovisions necessary to keep the NOx levels under control at therequired elevated temperatures. Furthermore, only the system includes asecond stage with enhanced swirl just for slag production. Thisconfiguration can also provide the option of converting existing ash toslag, thereby cleaning up the toxic ash pits that are common in coalburning regions. The second stage slag producer is mentioned above, withreference to FIG. 5.

Finally, while other systems produce slag in more or less sphericalshapes, the system has two unique slag formation features. First, theslag is extruded through a rectangular port that forms slag blocks uponsolidification. These blocks are thus quite stable at the disposal site,unlike conventional slag that can shift and thus pose a threat toworkers or any subsequent users of the disposal site.

Of course, the slag is molten when it passes through the extrusion mold.To maintain the rectangular shape, the slag must therefore be quenched.In the system, this quenching is done in a conventional water bath. Thenet effect is thus similar to lava from a volcano entering the sea. Likelava, the slag produces immense clouds of positively charged steam. Inthe system, this positively charged steam is then used to trap small ashparticles in a vapor trap. A suitable vapor trap is disclosed incopending U.S. application Ser. No. 13/471,918, herein incorporated byreference.

The system may be used with oxygen rich combustion. High oxygen levelsare useful for two reasons. First, coal and other poor quality fuels canundergo “gasification” to produce “synthetic gas” or “syngas,” which isthen burned or used as a feedstock. Although gasification has been usedfor years, only the present system provides separate combustion stages.This allows the control of the competing volatile and char reactions, asdescribed above. In particular, the system can provide either oxygen ornormal air to either stage as desired. For example, relatively cheap aircan be used to drive out the volatiles, leaving the char forgasification. An immediate extension of this concept is the control ofthe air and/or oxygen levels to produce coke in the second stage.Alternatively, oxygen can be added in the volatile stage, depending onthe chemistry of the specific coal type (the volatile composition andpercent varies with the coal type). In either case, the uniformity ofthe second stage char provides an easily controlled system.

An emerging use of this feature is the ability to burn coal underconditions of minimal nitrogen, as needed for various proposed “carboncapture and sequestration” (CCS) technologies. Although thesetechnologies are still under development, the ability of the system tooperate smoothly and reliably under initial high oxygen concentrationsis a great advantage over conventional systems.

Another option is the use of sound. As discussed in the first stageabove, and the third stage below, sound greatly aids combustionefficiency and stability. All of these advantages likewise apply here.The char combustor, however, also uses sound for slag control. Theunderlying problem is that slag varies greatly from coal seam to coalseam, because the mineral matter varies from seam to seam. Thisvariation is so great that some seams are described as “slagging” whileothers are described as “non-slagging.” The physical difference is thatsome slags flow freely, while others are so viscous that they accumulatein the combustor. The application of sound, however, causes even themost viscous slag to flow to the exit ports, thus allowing the system tofunction even with “non-slagging” coals.

3. Residual Gas and Particle Combustion Chamber

The products of the two previous modules 10, 20 are mainly gases, withsmall particles entrained in the flow. This third module burns thesegases and entrained particles, and thus completes the combustionprocess. The output of this module is thus essentially completecombustion of all carbon compounds, notably soot, volatile organiccompounds (VOC) and highly carcinogenic species, such as dioxins andfurans. The exhaust gas from the third stage, after cooling during thesteam generation process, is thus immediately ready for the subsequentvapor trap module. In particular, the output does not contain the largeamounts of particulates that are produced in conventional systems,thereby greatly reducing or even eliminating the need for downstreamparticulate control technology.

One way of producing highly effective combustion at low temperatures(for low NOx generation) is the use of a fluidized bed. The operatingprinciple of a fluidized bed is that an upflowing stream of combustionair supports the burning solid fuel. The result is thorough combustiondue to high turbulence and complete mixing. A common enhancement is toadd sand, limestone, or other non-combusting solids to the bed to reducepeak temperatures and capture pollutants, notably sulfur oxides.

There are several types of fluidized beds. Stationary or bubbling bedsuse low air velocity to suspend larger particles in an essentially fixedbed. Smaller particles are entrained into the exhaust. Recirculatingbeds use high air velocity to suspend and eventually entrain particlesof all sizes. A cyclone separator then grades the entrained particles bysize. Unburned particles are then returned to the bed to completecombustion, while spent particles are removed from the system.Stationary beds have the advantages of lower power to drive the air,reduced erosion on heat exchanger pipes, and entrainment of only smallerparticles, thereby reducing the need for downstream cyclones, filters,or other particle collection devices.

As noted earlier, coal has multiple combustion steps. In a conventionalfluidized bed, the coal shrinks as it burns, and thus progresses upthrough the bed while ash is released. The bed must therefore be deep.There are also problems with ignition, flame stability (sometimesrequiring a week or more to reach steady operation), and ash removal.

Conversely, in the residual gas and particle combustion chamber of thesystem, the fuel is mostly gaseous, the volatiles having been generatedin the devolatilization chamber, and the off-gas (mostly CO) generatedin the char combustor. The only solids are microscopic, and they flowalong with these gases. This configuration allows for thin beds that areeasy to build and require only low flow velocity to obtain fluidization.The immediate operational benefits are easier starting, rapid attainmentof operating temperature, stable operation, and no ash removal problems.

The gases coming from the volatile generator and the char combustor arevery hot. Heat exchanger tubes can be placed in these streams before thegases enter the fluidized bed combustor. The immediate design benefitsof placing the tubes outside the bed are decreased erosion, decreasedthermal NOx, enhanced space utilization, and lower net heat stress. Theoperational benefits are the options of using the tubes as (1)preheaters for the subsequent fluidized bed, or (2) post-heaters toprovide the high temperatures required for ultra-supercritical steamsystems, as required for the newest, highest efficiency turbineconfigurations.

A common procedure in coal fired fluidized beds is to use limestone as anon-combusting additive to reduce NOx (standard low temperaturecombustion technique) and trap sulfur oxides. In particular, thereaction of limestone and sulfur oxides yields synthetic gypsum, acommercially valuable product.

This approach is used with the limestone being ground before enteringthe bed. Grinding the limestone produces many particles that are toosmall to be confined in the fluidized bed. If these particles are addedto the bed, they will increase the particle loading on the downstreamparticle traps (ESP, filters, or the vapor trap described below). Thisadditional loading must be avoided because it: (1) decreases totalparticle collection, (2) increases processing costs, and (3) adds to thevolume, and thus the cost, of final disposal. It is therefore preferableto blow air through the ground limestone before it enters the bed. Theseparated large particles then proceed to the bed, while the fine dustis collected and sold for agricultural purposes.

Conventional fluidized beds are typically cylindrical in cross sectionto provide uniform flow and combustion. The gas fuel of the invention,however, enables the use of other geometries. Specifically, arectangular geometry is useful, with the incoming limestone fed in alongone side. The limestone then progresses uniformly across the bed, whereit exits on the opposite side from the entrance. This geometry providesuniform conversion of the incoming pure limestone to completelyprocessed gypsum at the exit. Additional enhancements include theaddition of flow straightening screens or grids to keep the flow uniform(plug versus laminar), as well as gradual deepening of the bed towardsthe exit to provide complete sulfur oxide capture even with partiallyprocessed limestone.

Conventional synthetic gypsum has several problems. First, when formedin a limestone scrubber, conventional gypsum is wet, and thereforerequires expensive drying before it can be used. Conversely, the gypsumin the present process is dry, and thus ready for use. Another problemis that conventional synthetic gypsum is formed at low temperatures, andat low flow rates, and therefore contains mercury and selenium, both ofwhich are toxic. In the system, these toxic elements are removeddownstream, and thus do not contaminate the gypsum. Finally,conventional synthetic gypsum can contain large amounts of ash,depending on the specific plant technology. Conversely, gypsum from thesystem is essentially ash free. The net result is that gypsum from thesystem has major market advantages over gypsum from conventionalsystems.

As noted, sound greatly influences combusting systems. Three approachesare utilized with the invention. Low frequency (low hundreds of Hz)oscillations move the inert bed particles, any incoming (small)particles, and the burning gases. These sound waves are applied from thesides of the bed, and propagate parallel to the bed. Because theparticles are suspended by the upflowing fluidizing air, these soundwaves are easily capable of displacing the particles in the horizontaldirection. The net result is great improvements in mixing andturbulence, and thus great improvements in combustion. Upward waves arenot helpful here because they would lift the entire bed.

Mid frequency (low kHz) move particles relative to gas. These soundwaves are applied vertically downward to compress the bed, and toconfine large particles to the bed. In any fluidized bed, the particlesnear the top are loosely distributed in a “freeboard.” The bed in thisregion thus performs more like an entrained flow system, which is notdesired. By known acoustic radiation pressure principles, the downwardforce is greater than the returning wave pulse, so that the net resultis the desired confinement. Once confined, any large fuel particles areburned out completely. Large inert particles are confined to the bed,and therefore proceed immediately to the gypsum trap. Only fineparticles can pass through the sound, thereby reducing the load on thedownstream particle traps to a minimum.

For comparison, centrifugal force systems have been used to obtain thesebenefits. The advantages of the sonic system over centrifugal systemsare (1) greatly reduced cost, (2) simpler operation, and (3) selectivesuppression with frequency control.

High frequency sound (Upper tens of kHz) moves the gas past theparticles, and thus aids combustion.

All three frequencies can be used together, just one at a time or anycombinations of any two.

To protect the sound sources from excessive heat, the sound is generatedaway from the bed and piped in using a “wave guide” technique asmarketed in a Bose stereo system. This approach also provides a means oftreating large beds, without excessive sound attenuation.

A particular advantage of mixed frequencies is enhanced heat transfer inthe tubes. The limiting problem here is that a boundary layer of gasforms around the tubes, thus reducing heat transfer. The application ofsound disrupts this boundary layer, while also increasing local mixing.The net result is improved heat transfer to the tubes, thus improvingoverall system efficiency without increasing erosion or causing othertypes of tube damage. The required sound can be generated by speakers orby horns. Additional enhancements include focused sound, as well asselected dampening to control excess noise.

An example of speakers 34 used with a fluidized bed 30 is depicted inFIG. 7. The speakers are positioned above the fluidized bed and belowthe outlet 32. The speakers 34 are phase linked with respect to distanceand time using standard wave techniques and coupled to the naturalresonance frequency of the reactor vessel to avoid destructiveinterference and promote constructive interference.

Speakers are useful along the entire height of the bed, beginning at thebase of the fluidized bed. The sonic waves agitate the bed to improveperformance and do not suffer from the disadvantages of mechanicalvibrators. The speaker system is inexpensive and the vibrations cantraverse the entire bed, thereby providing maximum, uniform treatment ofthe entire reactor vessel. The active component of this sound ishorizontal. The frequency is in the low hundreds of Hz. Naturalfrequency resonance is acceptable, and unavoidable, for standing waves.Traveling wave components, with appropriate phase linking, eliminateundesirable “dead zones” near the nodal points.

4. Combined System

FIG. 8 shows the above three components combined to form a completesystem. Raw coal, or other low quality organic fuel, enters thedevolatilization chamber 10. The intense heat and high turbulence inthis chamber cause rapid devolatilization. Optional enhancements includemicrowave energy pumping, sonic mixing, and high oxygen concentrationfeeding. In all cases, the total oxygen level is kept low to promotegasification and to prevent excessive burning of the volatiles. Thiscomponent yields two main output streams: (1) gaseous volatiles andcombustion products (both particulates and gases), which are then sentto the residual gas and particulate combustion chamber 30, and (2) char,which is then sent to the char combustor 20.

The char combustor, seen in FIGS. 4 and 5, receives the char from thedevolatilization chamber 10 entrained in a small amount of the gasproduced in the devolatilization chamber. This char is then mixed withair and burned. Optional enhancements include sonic mixing and highoxygen concentrations. In all cases, the total oxygen content is keptlow to decrease thermal NOx formation and to yield the maximum possibleconversion of char to gas. The ash is collected in either powder or slagform. The slag option includes remediation of existing sites, as well asthe formation of charged steam droplets to aid subsequent particulatecapture in a vapor trap. The final product is combustion gas, with theoption of water to shift the product to “syngas.” This stream alsoincludes small particulates.

The products from the devolatilization chamber and the char combustorare then collected and burned in the residual gas and particulatecombustion chamber 30. This chamber may be a fluidized bed, preferablyusing limestone to convert sulfur oxides to synthetic gypsum. One optionis the placement of heat transfer tubes before the residual gas andparticulate combustion chamber 30 to control peak temperatures, decreaseerosion, and improve overall system efficiency. Another option is sonicenhancement for improved bed performance and ideal combustion. Theproducts of this chamber are: (1) high pressure steam to drive turbinesor other equipment, and (2) a low pollutant exhaust stream that can bereleased immediately or subjected to additional cleaning.

FIG. 9 depicts a conventional system, such as that depicted in FIG. 2,retrofitted with a three stage combustor described above. As can beseen, just by replacing a conventional combustor with a three stagecombustor, acidic gases are decreased and the particle size is reduced,thereby relieving the load on downstream filters and other cleaningequipment. Likewise, FIG. 10 discloses a system integrating a threestage combustor. The electrostatic precipitator, NOx treatment and SOxscrubber are replaced with a particle trap. The particle trap uses waterdroplets to entrap particles. In addition, ozone is soluble in water,allowing the water droplets to remove any ozone in the exhaust stream.

While the invention has been described with reference to preferredembodiments, variations and modifications would be apparent to tone ofordinary skill in the art. The invention encompasses such variations andmodifications.

1. A combustor, comprising: a devolatilization chamber receiving a fuelsource, the devolatilization chamber removing volatile gases from thefuel source to create char; a char chamber receiving the char from thedevolatilization chamber and creating gases and ash; and a gas andparticulate combustion chamber receiving the volatile gases from thedevolatilization chamber and the gases from the char chamber.
 2. Thecombustor of claim 1, wherein the char chamber is a cyclone combustor.3. The combustor of claim 1, wherein the char chamber melts the ash intoslag.
 4. The combustor of claim 1 wherein the gas and particulatecombustion chamber is a fluidized bed.
 5. The combustor of claim 1,further comprising a microwave antennae in the char chamber to applymicrowave energy to the contents of the char chamber.
 6. The combustorof claim 1, further comprising at least one sonic driver in the charchamber to apply sonic energy to the contents of the char chamber. 7.The combustor of claim 1, further comprising a plurality ofdevolatilization chambers connected to the char chamber.
 8. Adevolatilization chamber comprising: a first end, a second end and aside wall extending between the first end and second end; an inlet inthe first end for receiving a fuel source; a first section forming aquarl and recirculation zone; a second section forming a charextraction/volatile generation zone; a side port formed in the sidewall; and an end port formed in the second end.
 9. The devolatilizationchamber of claim 8, further comprising a restriction between the firstsection and second section, the restriction having a smaller crosssectional area than the first and second sections.
 10. A method formanaging combustion, comprising: introducing a fuel-air mixture into adevolatilization chamber and causing combustion of the fuel air mixture;producing char and volatile gases from the combustion; sending the charto a char chamber, the char chamber producing gases and ash from thechar; and sending the gases from both the devolatilization chamber andchar chamber to a gas and particulate combustion chamber.
 11. The methodof claim 10, further comprising applying microwave or sonic energy tothe char chamber.
 12. The method of claim 10, further comprising meltingthe ash into slag.
 13. The method of claim 12, further comprisingextruding the slag through a port in the char chamber and quenching theslag.
 14. The method of claim 13, further comprising sending steamproduced from quenching to a vapor trap.